FIG. 6 is a side sectional view of a conventional semiconductor device. In that semiconductor device, a semiconductor element 1 is secured to a die pad 2 with an adhesive 3. Electrode pads of the semiconductor element 1 and internal leads 5 are electrically connected to each other by wires 4. The semiconductor element 1, the die pad 2, the adhesive 3, the wires 4, and the internal leads 5 are encapsulated in a sealing material 6, such as an epoxy resin. The internal leads 5 extend outside of the sealing material as external leads 7.
The total thickness of the sealing material, D, includes the thickness d.sub.1 of the semiconductor element 1 and the thickness d.sub.2 of the sealing material adjacent the die pad 2.
In the conventional semiconductor device, it is desirable to decrease the total thickness D. However, when the total thickness is reduced, the thickness d.sub.2 of the sealing material 6 is also reduced and the sealing material may not completely cover the die pad 2 so the quality of the encapsulated semiconductor device is poor.
It is also possible to decrease the thickness of the encapsulated semiconductor device by decreasing the thickness d.sub.1 of the semiconductor element 1. However, when the thickness d.sub.1 of the semiconductor element is decreased, the reliability of the semiconductor element declines.
Recent applications require semiconductor devices having good heat radiating characteristics. However, in the conventional semiconductor device shown in FIG. 6, good heat radiation characteristics cannot be obtained because the heat produced by the semiconductor element 1 is radiated through the sealing material 6. In order to improve the heat radiation characteristic, the entire reverse surface of the die pad 2 may be exposed, i.e., not covered by the sealing material, as illustrated in FIG. 7. In that structure, there is no sealing material under the die-pad 2, i.e., on the side of the die pad opposite the semiconductor element 1. Therefore, the total thickness D of the semiconductor device can be decreased by the thickness d.sub.2 so that the semiconductor device is thinner. In addition, the heat produced by the semiconductor element 1 is radiated through the exposed surface of the die pad 2. Thus, a good heat radiation characteristic is obtained.
However, when the entire reverse surface of the die pad 2 is exposed, stress concentrates on an end surface 2a of the die pad 2 at an interface between the die pad 2 and the sealing material 6. Separation can occur at that interface due to the stress concentration so that the reliability of the semiconductor device declines.